In resonant oscillating systems, such as found in many small appliances, including power toothbrushes, it is important that the drive frequency be quite close to the natural resonant frequency of the system for proper, efficient operation. The operation of such resonant frequency systems, however, can be significantly influenced by dynamic operational factors, including changes in loading on the device, e.g. loading of the brushhead in a power toothbrush, changes in the operating state of the power supply, i.e. the battery, and changes in performance of various components of the device due to aging or other factors. For instance, the resonant frequency of the system and the amplitude of movement of the device workpiece such as, for instance, a brushhead, can be affected by such changes.
One of the ways to accomplish a close match between the drive frequency for the device and the natural resonant frequency of the system-as well as maintaining performance of the device in the face of operating changes, such as those noted above, is to tightly control the tolerances of the system components. This approach, however, is expensive, and also may not provide a capability for compensating for changes in the system, either dynamic (short term), such as, for instance, changes in loading, or more permanent/long term, such as aging of the components.
An alternative approach is to use a sensing system which reads, i.e., determines, the actual operating frequency of the device and/or amplitude of movement of the workpiece during operation of the device and adjusts the frequency and power of the drive signal accordingly. U.S. Pat. No. 5,613,259 is an illustration of such an approach. However, the '259 approach requires a separate sensing system to monitor performance of the device and provide a signal to correct/change the drive signal accordingly. The separate sensing system adds expense to the device as well as complexity. Accordingly, it is desirable to have a sensing system which can make determinations of operational frequency and amplitude without the need for a separate sensing element.